It has been already known that tape pack loading or unloading operation is utilized to actuate a lift plate on which a head and a pinch roller are mounted to put them into positions for recording/reproducing mode or for achieving ejection of the tape pack. However, in the known mechanism, it is necessary to accurately put and lock the lift plate etc. in a position for the recording/reproducing mode at the last stage of the loading of the tape pack. To this end, it is required to interlock the lock mechanism at the last stage of the loading for establishing a precise locking. Thus, a considerably large operational force is required at the last stage of the loading where the retreat stroke of the tape pack left is small. Furthermore, over-push is generally required to accomplish the locking. For these reasons, it is difficult to establish the sure locking. In addition, a mechanism which is specifically adapted to attain such a sure locking generally has a drawback that there is a difficulty in release of the locking when ejection of the tape pack is desired.